Suspensions and isolation mounts generally fall into one of the following categories: passive, active or semi-active. Passive mounts usually include a passive spring and passive damper and can be tuned to provide very good isolation for a given set of conditions such as fixed masses and constant frequency disturbance into the unsprung mass. However if the mass changes due to increased payload, or the input frequency changes due to a change in speed over the ground, the isolation performance is degraded and often results in very large shock loads when the system hits the ends of travel, usually referred to as “topping” or “bottoming” the suspension.
Active suspensions are able to provide much better isolation over a wider range of conditions than a purely passive system. They can read a variety of sensors, then process the information to provide an optimal target force between the two masses at any time, given the power limits of the actuators and support systems. In addition, they are capable of adding energy to the system whereas passive and semi-active systems can only subtract energy. Active suspensions have not gained wide acceptance due to high cost and complexity as well as the demand for high power from the vehicles prime mover. In the case of off-road vehicles with long travel suspensions moving over rough terrain, the power draw of the suspension is prohibitive and reduces the maximum acceleration of the vehicle.
Semi-active suspensions are generally less costly and complex than fully active systems while retaining most of the performance advantages. They use the passive spring from conventional suspensions and add a controllable damper as well as the sensors and microprocessor required to allow the damper force to be controlled in real time. The damper can still only subtract energy from the system, however it can provide any level of damping that is demanded by the control method, rather than being governed by the fixed velocity/force laws that are characteristic of passive dampers.
There are a number of control methods that have been developed for semi-active suspensions, starting with “skyhook” method described by Karnopp, et al., “Vibration Control Using Semi-active Force Generator,” ASME Paper No. 73DET-123, May 1974, and U.S. Pat. No. 3,807,678. This method attempts to make the damper exert a force which is proportional to the absolute velocity of the sprung mass, rather than the relative velocity between the two masses. Hence the term skyhook since the mass is treated as though it is referenced to the inertial coordinate system rather than the ground. While this method can yield very good isolation over bumps that are smaller than the amount of compression travel in the system, larger bumps cause the suspension to bottom out resulting in a large shock load being transmitted into the sprung mass.
Another method has been developed to deal with the bottoming and topping problem called the “end stop” method. In end stop mode, the microprocessor calculates the minimum force required to decelerate the sprung mass and prevent the suspension from bottoming. While this is effective in preventing the high shock loads from being transmitted into the sprung mass, it results in excessive suspension movement over smaller bumps. This can be very disconcerting to the operator because it prevents him from having a good “feel” for the behavior and handling of the vehicle.
There have also been attempts to combine several methods and assign relative weightings or develop rules that govern the use of alternate methods under certain circumstances. Most of these efforts have been aimed at isolation efficiency as the overall goal or metric of relative merit. However there are other factors that are important in suspension systems such as transient force distribution that can influence handling and vehicle control, as well as subjective factors such as operator comfort and confidence.